1. The Field of the Invention
The present invention relates generally to conversion of biomass to fuels, fuel additives, and other commercially useful products. More particularly, the present invention relates to a multi-step catalytic process for production of hydrocarbon products from lignin.
2. Description of the Related Art
The current market for energy carriers and fuels is largely driven by high prices of petroleum and natural gas resulting from a depletion of easily accessible deposits, a growing demand caused by the development of new emerging market economies, and mounting environmental concerns. Consequently, these increasing energy demands will require a significant restructure and/or a replacement of a major portion of fossil fuels by renewable energy technologies such as biomass-based energy technologies. Mature technologies are available for the production of a variety of bio-commodities including transportation fuels and chemical building blocks from wood, agricultural crops, municipal solid wastes, landfill gas, etc., which will enable this new energy sector to evolve. This emerging sector serves as a network of bio-refineries (petrochemical refinery analogs) and represents an integrated, environmentally sound processing group of facilities. In these facilities biomass components are separated and converted into valuable intermediates and final products, including bio-fuels, bio-power, and other bio-products.
Cellulose and lignin represent two of the most prominent renewable carbon sources. Lignin, a second to cellulose as the most plentiful renewable carbon source on, Earth, is an amorphous three-dimensional energy-rich phenolic biopolymer, which is deposited in all vascular plants and provides rigidity and strength to their cell walls. The lignin polymeric structure is composed primarily of three phenylpropanoid building units: p-hydroxyphenylpropane (Structure I), guaiacylpropane (Structure II), and syringylpropane (Structure III) interconnected by etheric and carbon-to-carbon linkages. Generally, in unprocessed lignins, two thirds or more of these linkages are ether bonds, while the remaining linkages are carbon-carbon bonds.

Different types of lignin differ significantly in the ratio between these monomers. Inherent in its molecular nature, the lignin bio-mass component can potentially be converted directly to liquid fuels, e.g., high-octane alkylbenzene, aromatic ether gasoline-blending components, and/or napthenic kerosenes fuels (NK).
Currently, a limited supply of lignin is available as a by-product of the pulp and paper industry. However, in the near future, large quantities of lignin residue material will be available from biomass-to-ethanol processes and other biorefineries and associated processes. So far, in typical biorefinery process designs, lignin appears as a residual material with limited opportunities for its utilization. Other sources of lignin material can include agricultural products and wastes, municipal wastes, and the like.
Upgrading of the lignin residue by a catalytic conversion process to high-value fuels and fuel additives have been sought to enhance the competitiveness of biorefinery technologies. Numerous efforts on lignin conversion have included a number of single stage processing methods including hydrocracking, cracking, hydrogenation, hydrotreating, liquefaction in hydrogen-donor solvents, and the like. However, to date, these approaches have achieved limited success for a variety of reasons.
During the past few years several studies have been initiated to develop two-stage processes for making liquid fuels and fuel additives from lignin. These studies provided significant initial groundwork in identifying commercially useful processes for the conversion of lignin to valuable high-octane bio-fuels. Two methods are of particular interest and are described in U.S. Pat. Nos. 5,959,167 and 6,172,272, each of which are incorporated herein by reference in their entireties.
One of these methods is described as a lignin-to-gasoline (LTG) two-stage process. A first step involves base-catalyst depolymerization (BCD) of lignin feed in a reaction medium such as an alcohol followed by catalytic hydroprocessing (HPR). In the BCD stage of the process, lignin is partially depolymerized, mostly by solvolysis of etheric linkages, to ether-soluble mono-, bi- and trimeric phenolic units, and some hydrocarbons. In the second stage of the procedure, hydroprocessing (HPR), the BCD product is subjected to simultaneous or sequential “exhaustive” hydrodeoxygenation (HDO) and hydrocracking (HCR) for complete removal of remaining oxygen and to break the inter-aromatic C—C linkages. The overall BCD-HPR procedure yields a low-sulfur, high-octane hydrocarbon gasoline additive consisting of a mixture of C7-C11 alkylbenzenes, and some C6-C11 mono-, di-, tri- and polyalkylated naphthenes and C5-C11 (mostly multi-branched) paraffins (the latter resulting from by-products present in the lignin).
The second two-stage method is a lignin-to-aromatic ethers (LTE) process and was designed primarily for production of partially oxygenated gasoline or for the selective production of C7-C11 methoxybenzenes as high-octane additives. This process uses a similar two-stage procedure as the first process briefly described above. In the first stage, lignin is subjected to a mild base-catalyst depolymerization (BCD) in supercritical alcohol as a reaction medium. This step is then followed by non-deoxygenation/hydrotreatment/mildhydrocracking (HT), and a subsequent etherification (ETR) of the intermediate phenolic product to yield a reformulated, partially oxygenated gasoline. The resulting gasoline is typically a mixture of (substituted) phenyl methyl ethers (blending octane number, 124-166; boiling point, 154-195° C.) and cycloalkyl methyl ethers, C7-C10 alkybenzenes, C5-C10 (mostly multi-branched) paraffins, and polyalkylated cycloalkanes. Unfortunately, these two-stage processes can suffer from excessive catalyst coking and formation of polymeric solids which tend to clog the reactors. Further, alcohol-based solvents can alkylate with the reactants and oxidize to acetic acid which reduces yields and can make disposal of by-products more difficult. Accordingly, investigations continue into developing processes for conversion of lignin to more valuable products that are economically viable, especially aromatic gasoline components and napthenic products useful as jet and rocket fuel. Many challenges still remain to provide useful product yields without sacrificing process reliability